Best time-frame to be at the stream has been noon to 3PM. I prefer a bit of cloud cover because there is less solar-heating to dry the adults' wings; their wings have to be structurally sound before they alight off the water. The longer they drift, the more susceptible they are to be eaten.
NOTE: Unless specified the images are mine taken in Truckee area
Little Black Winter Stoneflies:
When they are about, you'll see them "peppering" the snow-banks; either crawling about as winged adults or as nymphs preparing to split their thoracic wing-case to emerge. Today they were fluttering at the water's surface in slow-running runs; otherwise they are very difficult to observe in the drift. Trout, generally, ingest these diminutive insects with gentle "slurps". Thin, 6X tippets are suggested for these size 16-18 aquatics. Consider two patterns; one that sits flush on the water's surface or a high-profile pattern with splayed wings, mimicking the top-water, fluttering adult (egg-layer?)
The existing BWO hatch is this year's "first-brood" emergence. I believe there may be three broods; winter, spring and fall...heresy? These small bugs are more visible while floating because they are "up-wings" as opposed to being a "down-wing" relative to the two stoneflies mentioned in this post. I prefer "old-school" Quigley Cripples, sparsely-tied ParaDun or CDC ComParaDun patterns; attached to minimum 10'-12' leader and a wispy 6X tippet. My first 20" RainBow of the 2020 season was fooled by the latter-mentioned pattern; size 18. Using the thin tippet, the hope is there are no obstructions while playing the trout.
A bit early, but we brought one to hand today. We anticipate a more robust emergence starting about the 3rd week of February. While at rest on the water they can be difficult to see being a "down-wing". The telltale of their presence is an aggressive swirl or bulge at the surface; unlike the gentle slurp for the Little Black Winter Stones. When the trout become aware of this stonefly adult, I like doubling my chances of a hook-up by trailing off the bend of the #8-10 Skwala dry a pattern such as a #18 JuJu Baetis or Flash-Back WD-40. A 5X tippet will suffice; being sturdy enough to "turn-over" the big fly fraud.
image by BugGuide.net